Semiconductor devices are used in a large number of electronic devices, such as computers, cell phones and others. One of the goals of the semiconductor industry is to continue shrinking the size and increasing the speed of individual devices. Smaller devices can operate at higher speeds since the physical gate length is smaller and follows the universal scaling law. In addition, higher conductivity materials, such as copper, are replacing lower conductivity materials, such as aluminum. As the geometries of semiconductor devices shrink and their performance improves, however, the maintaining low leakage current becomes a challenge.
One type of semiconductor device is a memory device, in which data is typically stored as a logical “1” or “0.” Memory devices may be static or dynamic. Dynamic memory devices need to be refreshed to “remember” the data, whereas static memory devices do not need to be refreshed to retain stored data.
One type of static memory device, also referred to in the art as a non-volatile memory (NVM) device, is a flash memory device. A flash memory device is an electrically erasable programmable read only memory (EEPROM) that is commonly used in computers, digital cameras, MP3 players, gaming systems, and memory sticks, as examples, although flash memory devices may be used in other applications, as well. Flash memory devices do not require power to retain stored data; they retain data even when the power source is disconnected. In flash memory devices, in-circuit wiring is used to erase predetermined sections or blocks of the chip by applying an electrical field to the entire chip, for example.
Flash memory devices typically comprise an array of flash memory cells. Flash memory cells are accessible for programming and retrieving data by an array of wordlines and bitlines coupled to the array of flash memory cells. Each flash memory cell comprises a floating gate and a control gate, which are separated by a thin insulator. Flash memory cells are programmed by applying a voltage to the control gate. Flash memory cells store a charge in the floating gate and are programmed using Fowler-Nordheim tunneling or channel hot electron injection from the channel or source and drain regions.
Recent flash memory applications include “embedded flash memory” and system on a chip (SoC) devices, in which an array of flash memory cells and peripheral circuitry for the flash memory cells are formed together with the microcontroller or processor on a single chip or integrated circuit. The peripheral circuitry may comprise high voltage circuits, for column/row decoders or drivers, charge pumps, transfer gates or other logic circuits for microcontrollers or processors, and other types of devices that may comprise transistors, diodes, bandgap devices, capacitors, inductors, and linear devices, as examples, although other types of devices may be included in the peripheral circuitry.
In the field of non-volatile memories (NVM), it is very desirable to shrink the size of the NVM cell so that either more memory can be stored in a given area to achieve higher density, or a given amount of memory takes up less silicon area to achieve more die per wafer to further reduce costs. Scaling down the size of the NVM cell requires reducing the length of the gate. This is typically achieved by thinning the tunnel oxide to control the short channel effect. A thinner tunnel oxide, however, leads to increased leakage current and loss of the stored charge on the floating gate. This negatively impacts the charge retention lifetime and the device reliability. What is needed is a method to prevent leakage current in NVMs while allowing for a reduced NVM cell size to take advantage of shrinking geometries.